i was gonna offer 80 too before Assia posted, but i havent seen any for sale in a while. honestly in an auction setting, with prices now adays they might end up getting closer to 200. myself, i wouldnt pay more than 100.
I dont know exactly where they came from, but they are a sirverbirth, and not an ultra rare one. Theres a decent amount out there but you dont see them for sale often. I also heard that there was a bug where you could get them from pirate ships for a while. Which is most likely a rumor which im sure someone here could confirm, or throw out.